Method for transmitting frame and communication device for performing the same

ABSTRACT

Disclosed are a frame transmission method and a communication device performing the same. The communication device may transmit a null data packet (NDP)-announcement (NDP-A) including information on a plurality of communication devices participating in interference alignment and transmit an NDP including a common signal field and a common training field commonly applied to the plurality of communication devices.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Korean PatentApplication No. 10-2015-0084007, filed on Jun. 15, 2015, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference.

BACKGROUND

1. Field of the Invention

Embodiments relate to a method of transmitting a frame and acommunication device for performing the same, and more particularly to amethod and a device allowing a plurality of communication devicesparticipating in interference alignment to simultaneously transmittraining symbols.

2. Description of the Related Art

In general, a local area network (LAN) that is a near fieldcommunication network is divided into a wired LAN and a wireless LAN(WLAN). The WLAN employs a method of performing communication on anetwork using propagation instead of using a cable. The WLAN has beenintroduced as an alternative for overcoming difficulties found ininstallation, maintenance and repair, and movement by cabling. Anincrease in mobile users is further accelerating a need for the WLAN.

A WLAN system includes an access point (AP) and a station (STA). The APis a device which transmits radio waves such that STAs within a servicerange access the Internet or use a network. The wireless LAN follows anInstitute of Electrical and Electronics Engineers (IEEE) 802.11 standardestablished by the IEEE.

A basic configuration block of a WLAN system defined in IEEE 802.11 is abasic service set (BSS). A BSS may include an independent BSS in whichSTAs in the BSS perform direct communications with each other, aninfrastructure BSS in which an AP is involved in communications betweenan STA and an STA inside or outside the BSS, and an extended service setwhich connects different BSSs to extend a service area.

Generally, an IEEE 802.11 based-WLAN system accesses a medium based oncarrier sense multiple access/collision avoidance (CSMA/CA) and allowsAPs to operate independently. That is, in the WLAN system, an APindependently selects a communication channel by an operator or achannel allocation algorithm. Thus, in the presence of multiple WLANsystems, communication channels used for respective BSSs are highlylikely to overlap. When communication channels overlap, possibility ofinterference occurring between adjacent BSSs is high, causingdeterioration in network performance. Thus, there is a need toefficiently reduce interference in a WLAN system.

SUMMARY

An aspect provides a frame transmission method which allows a pluralityof communication devices participating in interference alignment tosimultaneously transmit training symbols to effectively reduce overheadof preambles for channel information exchange.

Another aspect also provides a frame transmission method which allows aplurality of communication devices to transmit signal fields andtraining fields having the same values in the same structures tominimize interference which may occur when the communication devicessimultaneously transmit training symbols.

Still another aspect provides a frame transmission method whichdetermines a number of interference alignment-long training fields(IA-LTFs) included in an NDP for channel estimation and a number ofIA-LTFs included in a data field including data to be transmitted in thesame manner to effectively reduce a channel estimation error.

Yet another aspect provides a frame transmission method which determinesa number of IA-LTFs included in an NDP based on a number of streamstransmitted by a plurality of communication devices to effectivelyreduce interference between the streams from the communication devices.

According to an aspect, there is provided a frame transmission method ofa communication device, the method including transmitting a null datapacket (NDP)-announcement (NDP-A) including information on a pluralityof communication devices participating in interference alignment; andtransmitting an NDP including a common signal field and a commontraining field commonly applied to the plurality of communicationdevices.

The plurality of communication devices may perform interferencealignment using the common signal field and the common training field.

The common signal field and the common training field may include thesame values in the same structures as common signal fields and commontraining fields included in NDPs transmitted by the plurality ofcommunication devices.

A number of long training fields (LTFs) included in the common trainingfield may be determined based on a number of NDP streams transmitted bythe plurality of communication devices.

A number of LTFs included in the common training field may be determinedbased on a number of antennas of the plurality of communication devices.

A number of LTFs included in the common training field may be the sameas a number of LTFs included in each of common training fields of NDPstransmitted by the plurality of communication devices.

When a number of the plurality of communication devices participating ininterference alignment is predetermined, the NDP may include informationon a number of antennas of the plurality of communication devices, and astation receiving the NDP may estimate asignal-to-interference-plus-noise ratio (SINR) of a communicationchannel based on a number of antennas of a communication devicetransmitting the NDP and feed back the estimated SINR to thecommunication device transmitting the NDP.

When a number of the plurality of communication devices participating ininterference alignment is great, the NDP-A may include information on acombination of a plurality of communication devices capable ofparticipating in interference alignment, the NDP may include informationfor identifying a communication device transmitting the NDP, and astation receiving the NDP may estimate an SINR of a communicationchannel based on the information on the combination of the plurality ofcommunication devices capable of participating in interference alignmentand the information for identifying the communication devicetransmitting the NDP and feed back the estimated SINR to thecommunication device transmitting the NDP.

The method may further include transmitting a data frame including acommon signal field and a common training field commonly applied to theplurality of communication devices.

A number of LTFs included in the common training field of the data framemay be determined in the same manner as for determining a number of LTFsincluded in the common training field of the NDP.

A number of LTFs included in the common training field of the data framemay be determined based on a number of data frame streams transmitted bythe plurality of communication devices.

Common signal fields and common training fields included in data framestransmitted by the plurality of communication devices may have the samevalues.

The method may further include transmitting an NDP-poll includinginformation indicating that transmission of the NDP is completed,wherein the transmitting of the data frame transmits the data framebased on feedback information from a station receiving the NDP.

The transmitting of the NDP-A may transmit the NDP-A in a time periodwhen the plurality of communication devices transmits NDP-As.

The transmitting of the NDP may transmit the NDP in a time period whenthe plurality of communication devices transmits NDPs.

The transmitting of the NDP-A may transmit the NDP-A after an NDP-start(NDP-ST) including information indicating start of transmission of theNDP-A is received.

According to an aspect, there is provided a communication deviceincluding a communicator configured to communicate with a station; and aprocessor configured to control the communicator, wherein the processorperforms transmitting an NDP-A including information on a plurality ofcommunication devices participating in interference alignment; andtransmitting an NDP including a common signal field and a commontraining field commonly applied to the plurality of communicationdevices.

According to an embodiment, a plurality of communication devicesparticipating in interference alignment simultaneously transmitstraining symbols to effectively reduce overhead of preambles for channelinformation exchange.

According to an embodiment, a plurality of communication devicestransmits signal fields and training fields having the same values inthe same structures to minimize interference which may occur when thecommunication devices simultaneously transmit training symbols.

According to an embodiment, an IA-LTF) included in an NDP for channelestimation and an IA-LTF included in a data field including data to betransmitted are determined in the same manner to effectively reduce achannel estimation error.

According to an embodiment, a number of IA-LTFs included in an NDP isdetermined based on a number of streams transmitted by a plurality ofcommunication devices to effectively reduce interference between thestreams from the communication devices.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects, features, and advantages of the inventionwill become apparent and more readily appreciated from the followingdescription of embodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1 illustrates a basic service set (BSS) architecture in anoverlapping basic service set (OBSS) environment according to anembodiment;

FIG. 2 illustrates frames transmitted by communication devicesparticipating in interference alignment (IA) according to an embodiment;

FIG. 3 illustrates an example of a null data packet (NDP) in a physicallayer convergence procedure protocol data unit (PPDU) structuretransmitted from a plurality of communication devices participating inIA according to an embodiment;

FIG. 4 illustrates a method of determining a number of IA-Long TrainingFields (LTFs) included in an NDP according to an embodiment;

FIG. 5 illustrates an example structure of an IA-Signal Field-A(IA-SIG-A) when a number of communication devices participating in IA ispredetermined according to an embodiment;

FIG. 6 illustrates an example structure of an IA-SIG-A when there are agreat number of communication devices capable of participating in IAaccording to an embodiment;

FIG. 7 illustrates an example of a data frame in a PPDU structuretransmitted from communication devices participating in IA according toan embodiment;

FIG. 8 illustrates an example structure of an IA-SIG-B included in adata frame according to an embodiment;

FIG. 9 illustrates an example of a data frame transmitted from eachcommunication device when three communication devices participate in IAaccording to an embodiment;

FIG. 10 illustrates a detailed configuration of a communication deviceaccording to an embodiment;

FIGS. 11 and 12 illustrate a process of transmitting a stream accordingto an embodiment;

FIG. 13 illustrates a process of receiving a stream according to anembodiment; and

FIG. 14 illustrates a frame transmission method performed by acommunication device according to an embodiment.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. It isto be understood that the detailed description, which will be disclosedalong with the accompanying drawings, is intended to describe theexample embodiments of the present disclosure, and is not intended todescribe a unique embodiment with which the present disclosure can becarried out. The following detailed description includes detailedmatters to provide full understanding of the present disclosure.However, it will be apparent to those skilled in the art that thepresent disclosure can be carried out without the detailed matters.

The following embodiments are achieved by combination of structuralelements and features of the present disclosure in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present disclosure. The order of operations described in theembodiments of the present disclosure may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment.

Particular terms used in the following description are provided forunderstanding of the present disclosure and may be changed with otherterms without departing from the technical scope of the presentdisclosure.

In some cases, to prevent the concept of the present disclosure frombeing ambiguous, descriptions of known structures and apparatuses willbe omitted, or will be made in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, like reference numeralsdenote like elements through the whole document.

Embodiments of the present disclosure may be supported by standardsdisclosed in at least one of wireless access systems, such as anInstitute of Electrical and Electronics Engineers (IEEE) 802 system, a3rd Generation Partnership Project (3GPP) system, a 3GPP Long TermEvolution (LTE) system, an LTE-Advanced (LTE-A) system, and a 3GPP2system. That is, steps and parts not mentioned in the embodiments inorder to clarify the technical concept of the present disclosure may besupported by the foregoing standards. Further, all terms disclosed inthis document may be described by the standards.

Technologies described below may be used in various wireless accesssystems, such as Code Division Multiple Access (CDMA), FrequencyDivision Multiple Access (FDMA), Time Division Multiple Access (TDMA),Orthogonal Frequency Division Multiple Access (OFDMA), Single CarrierFrequency Division Multiple Access (SC-FDMA), and the like. CDMA may beimplemented with a radio technology, such as Universal Terrestrial RadioAccess (UTRA) or CDMA2000. TDMA may be implemented with a radiotechnology, such as Global System for Mobile communications(GSM)/General Packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA may be implemented with a radio technology, suchas IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved-UTRA(E-UTRA), and the like. Although the following description focuses on anIEEE 802.11 system for clarity, the technical concept of the presentdisclosure are not limited thereto.

FIG. 1 illustrates a basic service set (BSS) architecture in anoverlapping basic service set (OBSS) environment according to anembodiment.

A wireless local area network (WLAN) system may include at least oneBSS. The BSS may include an access point (AP) and at least one station(STA).

An AP is a functional entity which provides an STA associated with theAP with connection to a distributed system via a wireless medium. The APmay communicate with at least one STA on a downlink at a randomlydetermined moment.

A downlink is a communication link from an AP to STAs, and an uplink isa communication link from STAs to an AP. An STA may perform peer-to-peercommunication with another STA.

In a BSS including an AP, although communication between STAs isbasically performed via the AP, STAs may directly communicate with eachother not via the AP when direct links are established between the STAs.For example, the AP may also be referred to as other terms, such as acentral controller, a base station (BS), node-B, or a base transceiversystem (BTS), and be realized as the same.

A station (STA) may also be referred to as other terms, such as a mobileterminal, a wireless device, a wireless transmit/receive unit (WTRU),user equipment (UE), a mobile station (MS), a mobile subscriber unit orsimply a user, and be realized as the same.

An AP may simultaneously transmit data to an STA group including atleast one STA among a plurality of STAs associated with the AP. Here,“simultaneously” means “at the same time” or “at different times at apreset interval.” The WLAN system supports multi-user multiple-inputmultiple-output (MU-MIMO).

In an MU-MIMO communication system, an AP may transmit a plurality ofspatial streams to a plurality of STAs using multiple antennas. Further,when the AP uses a plurality of transmitting antennas, the AP maytransmit data frames to STAs using beamforming in order to improvetransmission performance. The AP may be referred to as a communicationdevice. In this specification, the terms “AP” and “communication device”may be used together for convenience of description.

In a wireless transmission environment of the WLAN system illustrated inFIG. 1, there are three communication devices and one STA is present percommunication device. As the communication devices are located withincoverage of another communication device, BSSs may overlap with eachother. Here, adjacent communication devices use different frequencychannels to prevent interference between adjacent BSSs. However, agradually increasing number of adjacent APs and STAs but limitedavailable frequency channels may cause an OBSS environment. That is, anOBSS environment may refer to an environment in which adjacentcommunication devices are caused to use the same frequency channel by agreat number of communication devices and STAs being concentrated butlimited frequency channels available for wireless communication.

An STA belonging to an OBSS environment may receive an undesired signalfrom an adjacent communication device. For example, STA2 may want toreceive a signal from AP1. However, STA2 is located within coverages ofother adjacent communication devices, AP2 and AP3, and thus may receivenot only a signal from AP1 but also signals from AP2 and AP3. STA2 mayreceive an interference signal from AP2 and AP3, the interference signalbeing represented by a dashed line in FIG. 1.

FIG. 2 illustrates frames transmitted by communication devicesparticipating in interference alignment according to an embodiment.

Any one of a plurality of communication devices participating ininterference alignment (IA) may be selected as a master communicationdevice (master AP) which is empowered to control operations of othercommunication devices. For example, the master communication device maybe a communication device capable of the most effectively monitoringstates of other communication devices among the plurality ofcommunication devices. An embodiment of selecting a master communicationdevice is not limited thereto, and various methods may be used to selecta master communication device.

The master communication device may transmit interference analignment-null data packet-start (IA-NDP-ST) to mark start of frametransmission. The communication devices may transmit an interferencealignment-null data packet-announcement (IA-NDP-A) after receiving theIA-NDP-ST. The IA-NDP-A may include information on the communicationdevices participating in IA (for example, a communication device list).Further, the IA-NDP-A may include information on at least one STArequired to perform channel feedback (for example, an STA list). TheIA-NDP-As transmitted from the communication devices may include thesame value in the same physical layer convergence procedure protocoldata unit (PPDU) structure. The communication devices may transmitIA-NDP-As in a non-high-throughput (HT) format in the same time period.

The communication devices may transmit an interference alignment-nulldata packet (IA-NDP) after transmitting the IA-NDP-A. The IA-NDP is aframe having no data field, which may include a signal field and atraining field. The IA-NDP may be transmitted via beamforming with atransmission vector predetermined by communication device or betransmitted without beamforming.

For convenience of description, a communication device performingcommunication with a particular STA among the communication devicesparticipating in IA is referred to as a reference communication device.An STA may receive an IA-NDP from the reference communication devicebased on an IA-NDP-A received from the reference communication device.The STA may transmit feedback information to the reference communicationdevice based on the IA-NDP from the reference communication device. Thecommunication devices may transmit IA-NDPs in the same time period.

The communication devices may transmit an interference alignment-nulldata packet-poll (IA-NDP-Poll) after transmitting the IA-NDPs. Thecommunication devices may transmit the IA-NDP-Poll, thereby explicitlynotifying the STA that a frame exchange sequence (for example,IA-NDP-ST, IA-NDP-A, IA-NDP, and the like) terminates. The communicationdevices may transmit IA-NDP-Polls in the same time period.

As illustrated in FIG. 2, the IA-NDP-ST, IA-NDP-A, and IA-NDP-Poll maybe transmitted repeatedly in each 20-MHz frequency band, and the IA-NDPmay be transmitted in an 80-MHz frequency band.

For convenience of description, hereinafter, an IA-NDP-ST may bereferred to as an NDP-ST, an IA-NDP-A as an NDP-A, an IA-NDP as an NDP,and an IA-NDP-Poll as an NDP-Poll.

FIG. 3 illustrates an example of an NDP in a PPDU structure transmittedfrom a plurality of communication devices participating in IA accordingto an embodiment.

The NDP in the PPDU structure is divided broadly into a common preambleand a dedicated preamble. The common preamble is a portion commonlyapplied to the communication devices participating in IA. That is,common preambles of NDPs transmitted from the communication devices mayinclude the same value in the same structure.

The common preamble includes a Legacy-Short Training Field (L-STF), aLegacy-Long Training Field (L-LTF), a Legacy-Signal Field (L-SIG), andan IA-Signal Field-A (IA-SIG-A). The L-STF, L-LTF, and L-SIG may includeinformation for a legacy STA capable of receiving a non-HT format. TheL-SIG may include an L-Length representing information on a length fromthe L-SIG to the IA-SIG-B.

The IA-SIG-A may include information on the communication devicesparticipating in IA. The information on the communication devices mayinclude information on which IA-LTF an STA receiving the NDP needs torefer to among a plurality of IA-LTFs included in the dedicatedpreamble. In the embodiment, the STA may need to refer to a plurality ofIA-LTFs. Also, the IA-SIG-A may include a list of the communicationdevices participating in IA and information on a number of antennasincluded in each communication device.

The dedicated preamble is a portion transmitted via beamforming with atransmission vector or matrix predetermined by communication device ortransmitted without beamforming. The dedicated preamble may include anIA-Short Training Field (IA-STF), an IA-Long Training Field (IA-LTF),and an IA-Signal Field-B (IA-SIG-B). The IA-STF may include informationfor automatic gain control (AGC) of an STA and signal detection, and theIA-LTF may include information for channel estimation for an STA andfrequency error estimation. A number of IA-LTFs may be determined basedon a number of antennas included in the communication devicesparticipating in IA. A method of determining a number of IA-LTFs will bedescribed in detail with reference to FIG. 4.

The IA-SIG-B may include information for identifying a communicationdevice transmitting the NDP. For example, the IA-SIG-B may include apattern or sequence for identifying a communication device transmittingthe NDP.

In FIG. 3, lengths of the NDP fields are expressed in microseconds (μs),which does not limit or restrict an embodiment applicable to lengths ofthe fields included in the NDP or the structure of the NDP.

FIG. 4 illustrates a method of determining a number of IA-LTFs includedin an NDP according to an embodiment.

FIG. 4 illustrates NDPs in a wireless transmission environment of threecommunication devices. Here, a first communication device (AP1) mayinclude two antennas, a second communication device (AP2) may includetwo antennas, and a third communication device (AP3) may include fourantennas. Each of the communication devices may transmit the same numberof NDP streams as a number of antennas included in each communicationdevice.

Each of the NDPs may include a legacy training field 410, a commonsignal field 420, a common training field 430, and a dedicated signalfield 440. The legacy training field 410 and the common signal field 420are constituent fields of a common preamble and may commonly be appliedto the communication devices. The legacy training field 410 and thecommon signal field 420 included in the NDPs transmitted from thecommunication devices may have the same value in the same structure.

The common training field 430 and the dedicated signal field 440 areconstituent fields of a dedicated preamble and may be transmitted viabeamforming with a transmission vector or matrix predetermined by acommunication device transmitting a corresponding NDP or be transmittedwithout beamforming. The common training field 430 may include an IA-STFand a plurality of IA-LTFs. The NDPs transmitted from the communicationdevices may include the common training field 430 including the samevalue in the same structure.

The dedicated signal field 440 may include information for identifying acommunication device transmitting a corresponding NDP among thecommunication devices. For example, NDPs transmitted from antenna 1 andantenna 2 of the first communication device (AP1) may include anIA-SIG-B in the same pattern or sequence. However, the IA-SIG-Bsincluded in the NDPs transmitted from the first communication device(AP1) may have a different pattern or sequence from IA-SIG-Bs of NDPstransmitted from the second communication device (AP2) or the thirdcommunication device (AP3).

For example, when a sequence included in an IA-SIG-B has a length of 26,the IA-SIG-B of the first communication device (AP1) may have a sequenceof 1011_1011_0011_1010_1101_1001_11, an IA-SIG-B of the secondcommunication device (AP2) may have a sequence of the1101_1001_1011_1011_0011_1010_10, and an IA-SIG-B of the thirdcommunication device (AP3) may have a sequence of0011_1010_1101_1001_1011_1011_01.

A number of IA-LTFs included in the common training field 430 of the NDPmay be determined based on a number of antennas of the communicationdevices participating in IA. Here, the antennas of the communicationdevices may refer to transmitting antennas used to transmit a frame toan STA.

A mapping code (or mapping matrix) P_(IALTF) mapped to an IA-LTFsequence may be determined based on a number of transmitting antennas ofa communication device as follows.

$\begin{matrix}{P_{IALTF} = \left. \overset{\overset{Symbols}{\rightarrow}}{\begin{bmatrix}c_{0,0} & c_{0,1} & \ldots & c_{0,m} \\c_{1,0} & c_{1,1} & \ldots & c_{1,m} \\\vdots & \vdots & \ddots & \vdots \\c_{n,0} & c_{n,1} & \ldots & c_{n,m}\end{bmatrix}}\downarrow\begin{matrix}{{Total}\mspace{14mu} {number}\mspace{14mu} {of}} \\{{Spatial}\mspace{14mu} {Streams}} \\{\left( {{or}\mspace{14mu} {antennas}}\; \right)\mspace{14mu} {for}} \\{Aps}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, n represents a number of streams of an NDP transmitted bya plurality of communication devices involved in IA, which may be thesame as a number of antennas of the communication devices. m representsa number of symbols, which may be determined based on the number ofstreams of the NDP transmitted by the communication devices. A detaileddescription of determining m will be described below with reference toEquations 3 to 5.

$\begin{matrix}{{{{AP}\; 1\text{:}\mspace{14mu} P_{{IALTF},1}} = \left. \overset{\overset{Symbols}{\rightarrow}}{\begin{bmatrix}c_{0,0} & c_{0,1} & \ldots & c_{0,m} \\c_{1,0} & c_{1,1} & \ldots & c_{1,m}\end{bmatrix}}\downarrow\begin{matrix}{{Number}\mspace{11mu} {of}\mspace{14mu} {spatial}\mspace{14mu} {streams}} \\{\left( {{or}\mspace{14mu} {antennas}}\; \right)\mspace{14mu} {for}\mspace{14mu} {AP}\; 1}\end{matrix} \right.}{{{AP}\; 2\text{:}\mspace{14mu} P_{{IALTF},2}} = {\begin{bmatrix}c_{2,0} & c_{2,1} & \ldots & c_{2,m}\end{bmatrix}{\quad{{\left. \downarrow\begin{matrix}{{Number}\mspace{11mu} {of}\mspace{14mu} {spatial}\mspace{14mu} {streams}} \\{\left( {{or}\mspace{14mu} {antennas}}\; \right)\mspace{14mu} {for}\mspace{14mu} {AP}\; 2}\end{matrix} \right.{AP}\; 3\text{:}\mspace{14mu} P_{{IALTF},3}} = {\begin{bmatrix}c_{3,0} & c_{3,1} & \ldots & c_{3,m} \\c_{4,0} & c_{4,1} & \ldots & c_{4,m}\end{bmatrix}{\quad\left. \downarrow\begin{matrix}{{Number}\mspace{11mu} {of}\mspace{14mu} {spatial}\mspace{14mu} {streams}} \\{\left( {{or}\mspace{14mu} {antennas}}\; \right)\mspace{14mu} {for}\mspace{14mu} {AP}\; 3}\end{matrix} \right.}}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Equation 2 illustrates mapping matrices P_(IALTF,1), P_(IALTF,2), andP_(IALTF,3) when the first communication device (AP1) transmits two NDPstreams, the second communication device (AP2) transmits one NDP stream,and the third communication device (AP3) transmits two NDP streams. Thenumber of symbols, m, may be greater than a number of streams to betransmitted according to a configuration of a mapping code in Equation 3to Equation 5.

Since the first communication device (AP1) includes two antennas andtransmits two NDP streams, a first mapping matrix P_(IALTF,1) may beallocated first two rows. Since the second communication device (AP2)includes one antenna and transmits one NDP stream, a second mappingmatrix P_(IALTF,2) may be allocated one subsequent row. Since the thirdcommunication device (AP3) includes two antennas and transmits two NDPstreams, a third mapping matrix P_(IALTF,3) may be allocated twosubsequent rows.

Since a symbol in a mapping code is mapped to an IA-LTF included in anNDP, a number of symbols in the mapping code may be the same as a numberof IA-LTFs included in the NDP. The number of IA-LTFs included in theNDP may be determined based on a number of NDP streams transmitted froma plurality of communication devices.

A mapping code is determined based on N_(IA-LTF), a number of NDPstreams transmitted from a plurality of communication devices. WhenN_(IA-LTF) satisfies 2^((n+1))<N_(IA-LTF)≦(3·2^(n)) (n=1, 2, . . . ), amapping code may be determined according to Equation 3. When N_(IA-LTF)satisfies N_(IA-LTF)≦2², a mapping code may be determined according toEquation 4. When N_(IA-LTF) satisfies (3·2^(n))<N_(IA-LTF)≦2^((n+2))(n=1, 2, . . . ), a mapping code may be determined according to Equation5.

$\begin{matrix}\quad & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \\{P_{IALTF} = {P_{{({2\alpha})} \times {({2\alpha})}} = {\quad{\quad{\quad{\quad{\begin{bmatrix}1 & {- 1} & 1 & \ldots & 1 & 1 \\1 & {- w^{1}} & w^{2} & \ldots & w^{{2\alpha} - 2} & {- w^{{2\alpha} - 1}} \\1 & {- w^{2}} & w^{4} & \ldots & w^{2{({{2\alpha} - 2})}} & {- w^{2{({{2\alpha} - 1})}}} \\\vdots & \vdots & \vdots & \ddots & \vdots & \vdots \\1 & {- w^{{2\alpha} - 2}} & w^{2{({{2\alpha} - 2})}} & \ldots & w^{{({{2\alpha} - 2})}{({{2\alpha} - 2})}} & {- w^{{({{2\alpha} - 2})}{({{2\alpha} - 1})}}} \\1 & {- w^{{2\alpha} - 1}} & w^{2{({{2\alpha} - 1})}} & \ldots & w^{{({{2\alpha} - 1})}{({{2\alpha} - 2})}} & {- w^{{({{2\alpha} - 1})}{({{2\alpha} - 1})}}}\end{bmatrix}{\quad {{w = {\exp \left( {{{- {j2\pi}}/2}\alpha} \right)}},{\alpha = \left\lceil \frac{N_{{IA} - {LTF}}}{2} \right\rceil}}}}}}}}}} & \; \\{P_{IALTF} = {P_{4 \times 4} = \begin{bmatrix}1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & 1 & 1 & {- 1} \\1 & 1 & 1 & 1\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \\{P_{IALTF} = {P_{{(2^{({n + 2})})} \times {(2^{({n + 2})})}} = \begin{bmatrix}P_{{(2^{({n + 1})})} \times {(2^{({n + 1})})}} & P_{{(2^{({n + 1})})} \times {(2^{({n + 1})})}} \\P_{{(2^{({n + 1})})} \times {(2^{({n + 1})})}} & {- P_{{(2^{({n + 1})})} \times {(2^{({n + 1})})}}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Here, Equation 5 may represent a mapping code extended from Equation 4.

According to one embodiment, a number of IA-LTFs included in the commontraining fields 430 of the NDPs illustrated in FIG. 4 may be determinedas follows. A total number of antennas included in the communicationdevices illustrated in FIG. 4 is eight. When a total number of NDPstreams transmitted from the communication devices, N_(IA-LTF), iseight, which satisfies (3·2¹)<8≦2⁽¹⁺²⁾, (n=1), a mapping code may bedetermined based on Equation 5. Matrix dimensions may be 2⁽¹⁺²⁾according to Equation 5. Thus, the common training field 430 may includeeight IA-LTFs in total.

According to another embodiment, it may be assumed that the firstcommunication device transmits three NDP streams, the secondcommunication device transmits four NDP streams, and the thirdcommunication device transmits two NDP streams. When a total number ofNDP streams transmitted from the communication devices, N_(IA-LTF), isnine, which satisfies 2⁽²⁺¹⁾<9≦(3·2²), (n=²), a mapping code may bedetermined based on Equation 3. Matrix dimensions may be 2α according toEquation 3. Thus, the common training field 430 may include 10 IA-LTFsin total.

According to still another embodiment, it may be assumed that the firstcommunication device transmits five NDP streams, the secondcommunication device transmits four NDP streams, and the thirdcommunication device transmits six NDP streams. When a total number ofNDP streams transmitted from the communication devices, N_(IA-LTF), is15, which satisfies (3·2²)<15≦2⁽²⁺²⁾, (n=2), a mapping code may bedetermined based on Equation 5. Matrix dimensions may be 2⁽²⁺²⁾according to Equation 5. Thus, the common training field 430 may include16 IA-LTFs in total.

According to yet another embodiment, it may be assumed that the firstcommunication device, the second communication device, and the thirdcommunication device each transmit one NDP stream. When a total numberof NDP streams transmitted from the communication devices, N_(IA-LTF),is three, which satisfies 3≦2², a mapping code may be determined basedon Equation 4. Matrix dimensions may be 4 according to Equation 4. Thus,the common training field 430 may include four IA-LTFs in total.

FIG. 5 illustrates an example structure of an IA-SIG-A when a number ofcommunication devices participating in IA is predetermined according toan embodiment.

It may be assumed that each STA recognizes, through an NDP-A,information on a plurality of communication devices participating in IA.In this case, each of the communication devices may transmit a Number ofSpace Time Stream (NSTS) representing a number of streams to betransmitted by each communication device through an IA-SIG-A. When thecommunication devices transmit NDPs, the NSTS may represent a number ofNDP streams transmitted by communication device or a number of antennasincluded in each communication device. Alternatively, when thecommunication devices transmit data frames, the NSTS may represent anumber of data frames transmitted by communication device.

AP[1] NSTS, AP[2] NSTS, AP[3] NSTS, and AP[4] NSTS illustrated in FIG. 5may represents numbers of streams to be transmitted by a firstcommunication device, a second communication device, a thirdcommunication device, and a fourth communication device, respectively.

An STA receiving an NDP including an IA-SIG-A may estimate asignal-to-interference-plus-noise ratio (SINR) of a communicationchannel based on an NSTS of a communication device of the STA and feedback the estimated SINR to the reference communication device.

FIG. 6 illustrates an example structure of an IA-SIG-A when there are agreat number of communication devices capable of participating in IAaccording to an embodiment.

A number of bits expressed through an IA-SIG-A may be limited. Thus, anumber of communication devices allowed to transmit NDPs in the sametime period to receive feedback on channel information is also limited.As illustrated in FIG. 5, when the IA-SIG-A is capable of expressingfour communication devices, the IA-SIG-A of FIG. 5 has no problem inexpressing four or less communication devices participating in IA.However, when five or more communication devices participate in IA, theIA-SIG-A structure of FIG. 5 is incapable of sufficiently expressingfrom which communication device an NDP is transmitted. To solve thisproblem, the IA-SIG-A may include an AP_Selection ID representing acombination of predetermined communication devices.

A great number of communication devices capable of participating in IAmay mean that a number of communication devices participating in IA isgreater than a number of bits expressed by an IA-SIG-A included in anNDP. When there are a greater number of communication devices capable ofparticipating in IA, the communication devices may construct acombination of a plurality of communication devices capable ofparticipating in IA in advance. Information on the combination isincluded in the AP_Selection ID, and the communication devices maytransmit the AP_Selection ID to an STA using an NDP-A or another controlframe. In the NDP, information indicating which communication device theNDP is transmitted from may be included in the AP_Selection ID of theIA-SIG-A.

STAs may estimate an SINR of a communication channel based on an NSTS ofa reference communication device of the STAs through the AP_Selection IDof the IA-SIG-A of the received NDP and feed back the estimated SINR tothe reference communication device.

FIG. 7 illustrates an example of a data frame in a PPDU structuretransmitted from communication devices participating in IA according toan embodiment.

A plurality of communication devices may transmit data frames to STAscommunicating with the communication devices based on SINRs receivedfrom the STAs. The data frame illustrated in FIG. 7 may have the samestructure as the NDP illustrated in FIGS. 3 and 4 except for an IA-SIG-Band a dedicated data field added last in the frame.

The data frame in the PPDU structure is divided into a common preamble,a dedicated preamble, and a dedicated data field. The common preamble isa portion commonly applied to the communication devices participating inIA. That is, common preambles of NDPs transmitted from the communicationdevices may include the same value in the same structure. The commonpreamble includes an L-STF, an L-LTF, an L-SIG, and an IA-SIG-A.

The dedicated preamble and the dedicated data field are portionstransmitted via beamforming with a transmission vector or matrixpredetermined by communication device or transmitted withoutbeamforming. The dedicated preamble may include an IA-STF, an IA-LTF,and an IA-SIG-B. Here, the IA-SIG-B included in the dedicated preamblemay have a different structure from the NDP of FIG. 2 and, for example,have a structure illustrated in FIG. 8.

FIG. 9 illustrates an example of a data frame transmitted from eachcommunication device when three communication devices participate in IAaccording to an embodiment.

FIG. 9 illustrates data frames in a wireless transmission environment ofthree communication devices. Here, a first communication device (AP1)may transmit two data frame streams through two antennas, a secondcommunication device (AP2) may transmit one data frame stream throughone antenna, and a third communication device (AP3) may three data framestreams through two antennas.

A dedicated preamble and a dedicated data field of a data frame may betransmitted via beamforming with a transmission vector and matrixpredetermined by communication device or transmitted withoutbeamforming. Here, even in a case of the same communication device, thepredetermined transmission vector and matrix may have a different valuedepending on an STA receiving a data frame stream. An IA-SIG-B includesinformation for an STA receiving a data frame and thus may have adifferent value depending on an STA to which a communication devicetransmits a data frame.

FIG. 10 illustrates a detailed configuration of a communication deviceaccording to an embodiment.

Referring to FIG. 10, the communication device 1000 may include acommunicator 1010, a processor 1020, a memory 1030, and an RF Front End1040.

The communicator 1010 may include a transmitter 1011 and a receiver1012. The communicator 1010 may communicate with an STA through the RFFront End 1040.

The transmitter 1011 may transmit a stream to an STA and perform coding,puncturing, interleaving, mapping, modulation, inverse fast Fouriertransform (IFFT), spatial mapping, or the like, and a detailedconfiguration thereof is not limited.

The receiver 1012 may receive a stream from an STA and perform fastFourier transform (FFT), equalization, demapping, demodulation,deinterleaving, depuncturing, decoding, or the like, and a detailedconfiguration thereof is not limited.

The processor 1020 controls the communicator 1010. The processor 1020may process digital transmission and reception supported bycommunication standards and may control, for example, Protocol LayerConvergence Procedure (PLCP), Physical Medium Dependent (PMD),associated layer management, Medium Access Control (MAC) layer, or thelike.

The processor 1020 may construct an NDP-A including information on aplurality of communication devices participating in IA and transmit theNDP-A to an STA. Also, the processor 1020 may construct an NDP includinga common signal field and a common training field commonly applied to aplurality of communication devices and transmit the NDP to an STA.

The memory 1030 is a storage circuit, which may be configured as anappropriate combination of various logics, circuits, codes, or the like,and a configuration thereof is not limited.

The device structure illustrated in FIG. 10 may be applied to not onlythe communication device 1000 but also an STA performing communicationwith the communication device 1000.

FIGS. 11 and 12 illustrate a process of transmitting a stream accordingto an embodiment.

In FIG. 11, it is assumed that transmitters 1110 of STA 1 to STA Ksimultaneously transmit a plurality of transmission streams. First, aweight matrix calculator 1120 may calculate a weight matrix or weightvector to be applied to the transmission streams of STA 1 to STA K froman MIMO channel. The weight matrix calculator 1120 may determine STA 1to STA K to simultaneously transmit transmission streams. A spatialmapper 1130 may map the determined weight vectors to the transmissionstreams of STA 1 to STA K. An inverse discrete Fourier transform (IDFT)performer 1140 may perform IDFT on the mapped transmission streams. Aguard interval (GI) inserter 1150 may add a GI to the IDFT transmissionstreams and perform windowing. A radio frequency (RF) unit 1160 mayperform digital-to-analog (D/A) conversion on the transmission streamsand transmit the transmission streams through an RF.

FIG. 12 illustrates a transmitter 1200 of STA K. A scrambler 1210 mayscramble data to be transmitted. An encoder parser 1220 may parse asmany scrambled data as a number of forward error correction (FEC)encoders 1230. An FEC encoder 1230 may perform FEC encoding on theparsed data. A stream parser 1240 may parse as many encoded data as anumber of streams. An interleaver 1250 may interleave the parsed data. Aconstellation mapper 1260 may map the interleaved data using any one ofbinary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK),16 quadrature amplitude modulation (16QAM), 64QAM, and 128QAM. Aspatial-time block code (STBC) encoder 1270 may perform STBC on themapped data. A cyclic shift delay (CSD) performer 1390 may perform CSDon the encoded data and transmit the data to the spatial mapper 1130 ofFIG. 11.

FIG. 13 illustrates a process of receiving a stream according to anembodiment.

FIG. 13 illustrates a receiver 1300 of STA K. First, an RF unit 1310 mayreceive a stream passing through a radio channel and performanalog-to-digital (A/D) conversion on the stream. A GI remover 1320 mayperform carrier sensing, AGC, timing synchronization, frequency offsetestimation, or the like on the received stream to remove a GI. Adiscrete Fourier transform (DFT) performer 1330 may perform DFT on thestream. A channel estimator 1331 may estimate a channel based on an LTFof the DFT stream. An MIMO detector 1332 may demodulate data in a datafield of the DFT stream using a channel estimation result. A de-mapper1340 may convert the demodulated data into a soft value needed for FECdecoding. A de-interleaver 1350 may de-interleave the converted softvalue. A stream de-parser 1360 may parse the de-interleaved soft valueaccording to a number of FEC decoders 1370. An FEC decoder 1370 mayperform FEC decoding on the parsed soft value. A decoder parser 1380 mayparse the FEC-decoded soft value. A de-scrambler 1390 may performde-scrambling on the parsed soft value to reconstruct data received bySTA K.

FIG. 14 illustrates a frame transmission method performed by acommunication device according to an embodiment.

The frame transmission method of the communication device according tothe embodiment may be performed by a processor of the communicationdevice and include transmitting an NDP-ST including informationindicating start of transmission of an NDP-A in operation 1410,transmitting an NDP-A including information on a plurality ofcommunication devices participating in IA in operation 1420,transmitting an NDP including a common signal field and a commontraining field commonly applied to the plurality of communication devicein operation 1430, transmitting an NDP-Poll including informationindicating that transmission of the NDP is completed in operation 1440,and transmitting a data frame including data to be transmitted inoperation 1450. Here, operation 1410 may be performed by a communicationdevice selected as a master communication device among the communicationdevices. Operations 1420 to 1450 may be simultaneously performed by thecommunication devices participating in IA.

The detailed matters described above with reference to FIGS. 1 to 13 areapplied to the operations illustrated in FIG. 14, and thus detaileddescriptions of the operations are omitted.

The example embodiments described herein may be implemented usinghardware components and software components. For example, the hardwarecomponents may include microphones, amplifiers, band-pass filters, audioto digital convertors, and processing devices. A processing device maybe implemented using one or more general-purpose or special purposecomputers, such as, for example, a processor, a controller and anarithmetic logic unit, a digital signal processor, a microcomputer, afield programmable array, a programmable logic unit, a microprocessor orany other device capable of responding to and executing instructions ina defined manner. The processing device may run an operating system (OS)and one or more software applications that run on the OS. The processingdevice also may access, store, manipulate, process, and create data inresponse to execution of the software. For purpose of simplicity, thedescription of a processing device is used as singular; however, oneskilled in the art will appreciated that a processing device may includemultiple processing elements and multiple types of processing elements.For example, a processing device may include multiple processors or aprocessor and a controller. In addition, different processingconfigurations are possible, such a parallel processors.

The software may include a computer program, a piece of code, aninstruction, or some combination thereof, for independently orcollectively instructing or configuring the processing device to operateas desired. Software and data may be embodied permanently or temporarilyin any type of machine, component, physical or virtual equipment,computer storage medium or device, or in a propagated signal wavecapable of providing instructions or data to or being interpreted by theprocessing device. The software also may be distributed over networkcoupled computer systems so that the software is stored and executed ina distributed fashion. In particular, the software and data may bestored by one or more computer readable recording mediums.

The methods according to the above-described examples may be realized asprogram instructions implemented by various computers and be recorded innon-transitory computer-readable media. The media may also include,alone or in combination with the program instructions, data files, datastructures, and the like. The program instructions recorded in the mediamay be designed and configured specially for the embodiments or be knownand available to those skilled in computer software. Examples of thenon-transitory computer readable recording medium may include magneticmedia such as hard disks, floppy disks, and magnetic tape; optical mediasuch as CD ROM disks and DVDs; magneto-optical media such as flopticaldisks; and hardware devices that are specially configured to store andperform program instructions, such as read-only memory (ROM), randomaccess memory (RAM), flash memory, and the like. Examples of programinstructions include both machine codes, such as produced by a compiler,and higher level language codes that may be executed by the computerusing an interpreter. The described hardware devices may be configuredto act as one or more software modules in order to perform theoperations of the above-described example embodiments, or vice versa.

While a few example embodiments have been shown and described withreference to the accompanying drawings, it will be apparent to thoseskilled in the art that various modifications and variations can be madefrom the foregoing descriptions. For example, adequate effects may beachieved even if the foregoing processes and methods are carried out indifferent order than described above, and/or the aforementionedelements, such as systems, structures, devices, or circuits are combinedor coupled in different forms and modes than as described above or besubstituted or switched with other components or equivalents. Thus,other implementations, alternative embodiments and equivalents to theclaimed subject matter are construed as being within the appendedclaims.

What is claimed is:
 1. A frame transmission method of a communicationdevice, the method comprising: transmitting a null data packet(NDP)-announcement (NDP-A) comprising information on a plurality ofcommunication devices participating in interference alignment; andtransmitting an NDP comprising a common signal field and a commontraining field commonly applied to the plurality of communicationdevices.
 2. The method of claim 1, wherein the plurality ofcommunication devices performs interference alignment using the commonsignal field and the common training field.
 3. The method of claim 1,wherein the common signal field and the common training field comprisethe same values in the same structures as common signal fields andcommon training fields comprised in NDPs transmitted by the plurality ofcommunication devices.
 4. The method of claim 1, wherein a number oflong training fields (LTFs) comprised in the common training field isdetermined based on a number of NDP streams transmitted by the pluralityof communication devices.
 5. The method of claim 1, wherein a number ofLTFs comprised in the common training field is determined based on anumber of antennas of the plurality of communication devices.
 6. Themethod of claim 1, wherein a number of LTFs comprised in the commontraining field is the same as a number of LTFs comprised in each ofcommon training fields of NDPs transmitted by the plurality ofcommunication devices.
 7. The method of claim 1, wherein when a numberof the plurality of communication devices participating in interferencealignment is predetermined, the NDP comprises information on a number ofantennas of the plurality of communication devices, and a stationreceiving the NDP estimates a signal-to-interference-plus-noise ratio(SINR) of a communication channel based on a number of antennas of acommunication device transmitting the NDP and feeds back the estimatedSINR to the communication device transmitting the NDP.
 8. The method ofclaim 1, wherein when a number of the plurality of communication devicesparticipating in interference alignment is great, the NDP-A comprisesinformation on a combination of a plurality of communication devicescapable of participating in interference alignment, the NDP comprisesinformation for identifying a communication device transmitting the NDP,and a station receiving the NDP estimates an SINR of a communicationchannel based on the information on the combination of the plurality ofcommunication devices capable of participating in interference alignmentand the information for identifying the communication devicetransmitting the NDP and feeds back the estimated SINR to thecommunication device transmitting the NDP.
 9. The method of claim 1,further comprising transmitting a data frame comprising a common signalfield and a common training field commonly applied to the plurality ofcommunication devices.
 10. The method of claim 9, wherein a number ofLTFs comprised in the common training field of the data frame isdetermined in the same manner as for determining a number of LTFscomprised in the common training field of the NDP.
 11. The method ofclaim 9, wherein a number of LTFs comprised in the common training fieldof the data frame is determined based on a number of data frame streamstransmitted by the plurality of communication devices.
 12. The method ofclaim 9, wherein common signal fields and common training fieldscomprised in data frames transmitted by the plurality of communicationdevices have the same values.
 13. The method of claim 9, furthercomprising transmitting an NDP-poll comprising information indicatingthat transmission of the NDP is completed, wherein the transmitting ofthe data frame transmits the data frame based on feedback informationfrom a station receiving the NDP.
 14. The method of claim 1, wherein thetransmitting of the NDP-A transmits the NDP-A in a time period when theplurality of communication devices transmits NDP-As.
 15. The method ofclaim 1, wherein the transmitting of the NDP transmits the NDP in a timeperiod when the plurality of communication devices transmits NDPs. 16.The method of claim 1, wherein the transmitting of the NDP-A transmitsthe NDP-A after an NDP-start (NDP-ST) comprising information indicatingstart of transmission of the NDP-A is received.
 17. A communicationdevice comprising: a communicator configured to communicate with astation; and a processor configured to control the communicator, whereinthe processor performs transmitting a null data packet(NDP)-announcement (NDP-A) comprising information on a plurality ofcommunication devices participating in interference alignment; andtransmitting an NDP comprising a common signal field and a commontraining field commonly applied to the plurality of communicationdevices.
 18. The communication device of claim 17, wherein the pluralityof communication devices performs interference alignment using thecommon signal field and the common training field.
 19. The communicationdevice of claim 17, wherein the common signal field and the commontraining field comprise the same values in the same structures as commonsignal fields and common training fields comprised in NDPs transmittedby the plurality of communication devices.
 20. The communication deviceof claim 17, wherein a number of long training fields (LTFs) comprisedin the common training field is determined based on a number of NDPstreams transmitted by the plurality of communication devices.